text: # -*- coding: utf-8 -*-
  """
  Created on Mon Aug 14 15:45:36 2017
  
  @author: hexo
  """
  
  import pandas as pd
  
  import numpy as np
  
  a = pd.Series([np.nan,2.5,np.nan,3.5,4.5,np.nan]
                      ,index=['f','e','d','c','b','a'])
  
  b = pd.Series(np.arange(len(a),dtype=np.float64)
                      ,index=['f','e','d','c','b','a'])
  
  b[-1] = np.nan
  
  print(a)
  print(b)